Printing machines produce images on sheets or webs of paper and similar materials. When in sheet form, such materials are normally piled in the feeder of the machine in preparation for printing. Mechanism is then required for separating one sheet at a time from this pile, normally from the top, and for moving this sheet through the machine for application of the image, and possibly for other processing, after which the sheet is normally deposited on a second pile in the delivery of the machine. This process usually takes place continuously, with a subsequent sheet being separated from the feeder pile for printing while the previous sheet may still be in the final stages of the process of being printed and delivered.
In machines for printing high quality multicolor images, the sheet must be moved with high precision and repeatability of position and the means for holding the sheet in this movement must be adapted to the printing process. This is commonly accomplished by sets of grippers, holding the leading edge of the sheet, which are mounted either near the surface of a rotating transfer drum cylinder or are part of a moving gripper bar. To be adapted to the printing process, the grippers must be formed to occupy a relatively small space but nevertheless capable of exerting high forces on the leading edge of the sheet so as to avoid slippage during the process. As a result, the opening of these grippers, commonly referred to as a gripper throat, into which the leading edge of the sheet must be introduced after separation from the feeder pile, is commonly quite narrow. The mechanism for feeding the sheet into this gripper throat, commonly referred to as infeed mechanism, must therefore control the sheet in a precise manner so as to avoid misfeeding of the leading edge above or below this throat. This control is particularly important when there are wrinkles or curly portions in the leading edge of the sheet as commonly occurs in such applications.
Previous infeed mechanisms have been complex and mechanically unstable. In one of these mechanisms, the sheet is guided through a chute whose motion is governed by a four bar linkage mechanism. The accurate control of this linkage mechanism is difficult and an occasional paper misfeed occurs when the operation of the machine is extended to sheet stocks of diverse thickness, stiffness, surface texture, and curl, such as is required in many applications. One of the causes of malfunction is the variable surface friction of various sheet stocks which results in slight variations in the speed with which sheets are forwarded to the position where the leading edge is introduced into the gripper throat. Because of the resulting delays in sheet arrival the leading edge occasionally does not reach the gripper throat or, alternately the leading edge is gripped only partly during closure of the gripper fingers. The alignment of the chute in relation to the moving array of grippers, during the movement of the leading edge of the sheet into the gripper throat, has likewise proven to be unreliable and misfeeds have been experienced in this area. An improvement of this mechanism in which the same type of chute is controlled by a ball screw mechanism has not offered significant advantages, especially at the high process speeds which are commonly required in such machines.